alien
Well-Known Member
I just soldered a locknut on my keg using Stay Clean paste flux. I didn't get it right first time, so I thought I would share my technique.
Stay Clean liquid flux is generally recommended instead of paste flux. The fluxes are chemically similar in that they both contain zinc chloride and ammonium chloride. The main difference is that the paste flux has a base of petrolatum instead of water. This makes it easy to apply but it also makes it easy to burn and the clean up is messy. My local Airgas only had the paste flux (Radnor brand). It was $3 for 4 oz.
My first attempt involved fluxing both nut and keg, and trying to apply solder (4% Ag) directly to the join. I applied heat to the inside of the nut using a propane torch with a strong flame. But the solder balled and soon I had over heated the joint. The flux turned black and I had to remove the nut, sand everything down, and start again.
Second time around I decided to pre-tin the nut. To avoid heating the nut directly, I placed it on a scrap sheet of aluminium and applied the flame to the sheet. It took a couple of goes but eventually I had a layer of solder on the surface of the nut. The crucial thing was to stop heating the moment the nut was hot enough to melt solder. The aluminium plate acted as a heat sink to keep the temperature high.
After sanding and cleaning I applied flux to the keg and put the locknut over the hole. I heated much more gently applying the flame only to the inside thread of the nut. As soon as I saw liquid solder emerge I removed the flame. I touched up with solder around the nut, heating only as much as necessary to keep the solder liquid.
Finally, I cleaned up the joint. There were solder blobs here and there and some unsightly balling. I let the metal cool slightly, then brushed a little more flux around the joint. I turned the torch to a high flame and feathered it around the joint, holding the torch at a low angle so that it melted the solder but didn't apply much heat to the surface of the keg. Then I flicked away excess solder with the flux brush to make a neat join. (Not with the brush that came inside the flux pot, a Harbor Freight cheapie.)
While flicking the solder away I managed to knock the nut sideways and had to nudge it back in place. It didn't matter because both surfaces were already tinned. But I did have to let it cool slightly and go round again with the flux brush and torch to neaten things up.
By the way I did not flatten the keg wall or radius the nut. Silver bearing solder has a bit of a window between the solidus and liquidus so if you keep the temperature low enough you can build the solder joint up to cover the poor fit.
Overall it was pretty tricky. In the past I have used phosphoric acid for flux and found it much easier to work with. It is much more tolerant of excess heat and easier to clean up because it is water soluble and the phosphate residues, unlike chlorides, are non-corrosive.
Quick photo with the phone while it was cooling down. I'm going to clean up with warm soapy water, then with alcohol, and lastly with sodium bicarbonate solution to neutralize any residual flux.
Stay Clean liquid flux is generally recommended instead of paste flux. The fluxes are chemically similar in that they both contain zinc chloride and ammonium chloride. The main difference is that the paste flux has a base of petrolatum instead of water. This makes it easy to apply but it also makes it easy to burn and the clean up is messy. My local Airgas only had the paste flux (Radnor brand). It was $3 for 4 oz.
My first attempt involved fluxing both nut and keg, and trying to apply solder (4% Ag) directly to the join. I applied heat to the inside of the nut using a propane torch with a strong flame. But the solder balled and soon I had over heated the joint. The flux turned black and I had to remove the nut, sand everything down, and start again.
Second time around I decided to pre-tin the nut. To avoid heating the nut directly, I placed it on a scrap sheet of aluminium and applied the flame to the sheet. It took a couple of goes but eventually I had a layer of solder on the surface of the nut. The crucial thing was to stop heating the moment the nut was hot enough to melt solder. The aluminium plate acted as a heat sink to keep the temperature high.
After sanding and cleaning I applied flux to the keg and put the locknut over the hole. I heated much more gently applying the flame only to the inside thread of the nut. As soon as I saw liquid solder emerge I removed the flame. I touched up with solder around the nut, heating only as much as necessary to keep the solder liquid.
Finally, I cleaned up the joint. There were solder blobs here and there and some unsightly balling. I let the metal cool slightly, then brushed a little more flux around the joint. I turned the torch to a high flame and feathered it around the joint, holding the torch at a low angle so that it melted the solder but didn't apply much heat to the surface of the keg. Then I flicked away excess solder with the flux brush to make a neat join. (Not with the brush that came inside the flux pot, a Harbor Freight cheapie.)
While flicking the solder away I managed to knock the nut sideways and had to nudge it back in place. It didn't matter because both surfaces were already tinned. But I did have to let it cool slightly and go round again with the flux brush and torch to neaten things up.
By the way I did not flatten the keg wall or radius the nut. Silver bearing solder has a bit of a window between the solidus and liquidus so if you keep the temperature low enough you can build the solder joint up to cover the poor fit.
Overall it was pretty tricky. In the past I have used phosphoric acid for flux and found it much easier to work with. It is much more tolerant of excess heat and easier to clean up because it is water soluble and the phosphate residues, unlike chlorides, are non-corrosive.
Quick photo with the phone while it was cooling down. I'm going to clean up with warm soapy water, then with alcohol, and lastly with sodium bicarbonate solution to neutralize any residual flux.